An annealing procedure in the production of a planar wave guide cladding can be critical because the precursor materials for the deposition, i.e. the plasma creating substances (typically hydrides and nitrous oxide), are liable to create unwanted chemical substances such as radicals with bonded hydrogen in the films. The annealing procedure may expel the undesired elements in such a combination, but the sites of the expelled elements are then likely to contain voids, gaps and other imperfections in the structure of a deposited wave guide layer. These imperfections may not only detract from the strength and dimensional predictability of a wave guide so treated, but may lead to inhomogenities in its refractive index, thus prejudicing its optical operation. Since a typical operating wavelength may be 1.55 microns, and the operation of a planar wave guide may depend on such small differences in refractive index as 0.7% between the cladding and the core and the buffer on which it rests, even minor errors in refractive index are unacceptable.
Annealing tends to reduce any imperfections, and to consolidate or density the material deposited by a PECVD process, but an annealing procedure can take a long time, and arrangements to be described below provide a reliable improvement in a structure without the employment of an unduly long annealing time. Previously proposed methods of annealing deposited layers, especially thick layers usually involved slow heating in attempts to avoid the disadvantage, even when layers are produced by means other than a PECVD process, of a tendency to cause stresses and cracking. The films deposited by PECVD, low pressure chemical vapour deposition (LPCVD) or by atmospheric pressure chemical vapour deposition (APCVD) are coherently bonded, hard and stable, unlike the films deposited by previous flame hydrolysis techniques which are soot-like, soft and porous. The properties of these PECVD, LPCVD or APCVD deposited films provide a reduced tendency to cracking or blistering during an annealing process. The cracking probably results from a physical mismatch between the wave guide core, its substrate, the intermediate buffer layer which is usually present, especially with thick films, and the deposited cladding. With a PECVD process, the precursor materials used are of such a nature, as explained above that they tend to add to the problems of annealing.
Many materials such as those of interest herein can also be deposited in layers by spin coating from liquid sources generally referred to as Solgel. Such a technique is described for example by R A A Syms and A S Holmes in J. Non-crystalline Solids, 170, 1994, pp. 223 to 233. However, the resulting layers are severely limited in thickness, and each layer may take three minutes or so to produce. Therefore there is no substantial advantage in techniques for their rapid annealing. With the more rapid PECVD process, need for rapid annealing is much more significant.
Fast rate annealing of very thick (.gtoreq.15 micron) borophosphosilicate glass (BPSG) cladding layers which have been deposited using PECVD (the cladding oxide may be undoped or contain dopants such as germania) was carried out by first removing unstable products, such as bonded hydrogen and water vapours at low temperatures e.g. 700.degree. C. and then step by step raising the temperature till the cladding layer fills up the gaps between and around the etched core layers. The annealing time and temperatures are determined by the composition of the cladding layer. Annealing temperature-time combination is critical for controlling all the requisites for a low loss cladding layer such as refractive index homogeneity across the thickness of the film, and also around the channel wave guides.
As an example, one prior proposed cladding process consists of depositing 1, 3, 6 and 6 microns thick BPSG film by PECVD. Each of the layers is separately annealed. This prior proposed annealing is carried out by slowly ramping a resistively heated furnace containing the wafers, from 300.degree. C. to 1000 or 1050.degree. C. The total time for annealing each layer is around 16 hours, thus making the total time of anneal for a 15 microns thick layer up to 64 hours. The slow ramping was believed to be necessary to prevent cracking or degradation of the film, which may occur if the film is suddenly exposed to temperatures close to the flow temperatures of the BPSG. Such long annealing times are undesirable for manufacturing of planar wave guide devices, as this will slow down the product throughput.
Rapid annealing of phosphosilicate and borophosphosilicate glasses for the purposes of integrated circuit application has been described for example by R Thakur et al. in Proceedings of 11th VMIC Conference, California, Jun. 7-8, 1994, pp 117 to 119. The glass film, typically .ltoreq.2 microns in thickness in the case of IC technology is used as an interlayer dielectric where the key requirement of the material is its electrical insulation rather than any optical properties. In planar wave guide applications as envisaged herein, the glass thickness is not only significantly greater, typically .gtoreq.16 microns, but the control of the refractive index of the film across the substrate, and more importantly through the thickness of the film, needs to be accurate to about 0.002 of the refractive index value to provide a high quality device. An accurate control of this nature is not a feature of the processes employed to form an IC interlayer dielectric film.